The invention is related to a traveling-wave tube which has a cylindrical evacuated sheath which is surrounded by a permanent-magnet system having poles shoes with magnetic rings inserted between the pole shoes which are axially arranged with alternating opposite polarization. In addition, the pole shoes inserted in the evacuated sheath and the parts of them that surround the beam axis are designed as small tubes, with every second pole shoe coupled as an active pole shoe to the magnetic rings, and the pole shoes that are inserted between are connected with the evacuated sheath.
The focusing of an electron beam from a traveling-wave tube by means of a periodic permanent-magnet focusing system, mounted externally on the evacuated sheath of the tube, is a familiar process. Focusing systems of this kind usually consist of permanent magnet rings and pole shoes of ferromagnetic material inserted between them.
A pole shoe arrangement of this type which serves simultaneously as an evacuated sheath is disclosed in German Patent DE-AS No. 14 91 529. Simultaneous mechanical processing of all the internal borings of the pole shoes allows the pole shoes to be oriented exactly to the axis. In addition, the magnet rings are centered on the inner diameter.
In traveling-wave tubes with a very high output, a sturdy coupled cavity line is used with a large outer diameter. The field strength of a superimposed ring magnet system would therefore be too small to be able to focus electron beams with a high perveance, such as are needed for a high output. Therefore, the pole shoes are introduced into the tube by constructing the conducting discs as pole shoes ("integrated pole shoes"). Particularly suitable for this purpose is the coupled-cavity wiring with "caps" (i.e., the parts of the conducting discs that are close to the axis and have the shape of small tubes). FIG. 1 shows schematically a conventional system of this kind, and FIG. 2 shows graphically the magnetic field generated by such a system. There are specific reasons why every second conducting disc consists of an active pole shoe connected to the magnets. First, this achieves a compensation for the magnetic asymmetry caused by windows 10. Second, it produces a suppression of the ripple of the first order by means of the harmonics of the magnetic field. The ripple of the first order is either almost or completely suppressed by a field configuration of the type shown in FIG. 2. Through measurement of the delay line the ratio h/l (gap/cell length) is obtained, by means of which the magnetic design parameters can be determined.
The thickness t of the conducting discs sould be as small as possible, otherwise the coupling resistance in the area of the beam will be reduced by an unfavorable shift in the electric field. The limitation of this magnetic system is therefore the iron stress B.sub.1 on the disc, which reaches its highest value at Point B. It is essential for several reasons to prevent the iron stress from reaching the point of magnetic saturation, in particular to eliminate impermissible production spreads. Since the measurements of the magnetic system also determine B.sub.1 /B.sub.eff, the limitation on the iron stress has the effect of determining a limit for the effective fluid strength B.sub.eff. From the equilibrium relationship ##EQU1## and the relationship for the frequency ##EQU2## it follows that the perveance of the beam, P.sub.o and the frequency, f have an upward limit. (Units: 10.sup.-4 T, V, A, cm, GHz. U.sub.o is the beam voltage, .gamma. is the middle radius, .gamma.a is the phase parameter and K.sub.eff is the cathode field parameter). According to the state of the art, there are travelling-wave tubes of this kind with (approximately) P.sub.o =2.multidot.10.sup.-6 A/V.sup.3/2 for f=9 to 10 GHz. for conventional magnetic systems of this kind, known for example from U.S. Pat. No. 3,324,339 and shown in FIG. 1, the parts of the conducting discs that are close to the axis are designed as small tubes and consists of magnetic iron.